The present invention relates to a method for reducing the oxidation loss of chromium and the wear of refractories, by performing refining in accordance with a molten steel temperature and a [C] concentration, in decarburization-refining of chromium-contained steel under a normal or reduced pressure.
Methods for refining molten steel in accordance with a [C] concentration in the decarburization-refining of chromium-contained molten steel containing chromium by 11 mass % or more like stainless steel are know. One such method is a dilution-decarburizing method using which a CO gas partial pressure (PCO) in an atmosphere is reduced by injecting a dilution gas together with oxygen gas (hereunder referred to simply as xe2x80x9coxygenxe2x80x9d) on and after the middle period of decarburization (for example, when a [C] concentration is 0.7 mass % or less). Another such method is a vacuum-decarburizing method, wherein molten steel is charged into a ladle and treated with the interior of the ladle depressurized. The former method is generally called the AOD method or the top-and-bottom blown converter method and the latter method the VOD method.
Further, as described in Japanese Patent Publication Nos. H3-68713 and H4-254509, the vacuum AOD method is used with which depressurized refining is applied on the way of decarburization in the AOD method has been employed recently. In the vacuum AOD method, the amount of supplied oxygen and the degree of vacuum are adjusted in accordance with a [C] concentration in the same way as in the VOD method.
Any of these methods may be intended to efficiently promote decarburization while the oxidation loss of [Cr] in molten steel is suppressed. However, in any of such conventional methods, the increase of the oxidation of [Cr] in proportion to the decrease of a [C] concentration cannot be avoided and thus the amount of the oxidation of [Cr] increases.
To suppress the oxidation loss of [Cr] in molten steel, in the VOD method for example, the amount of supplied oxygen and the degree of vacuum (not more than 100 Torr) are hitherto adjusted in accordance with the progress of decarburization as described in Japanese Patent Publication Nos. S55-89417 and S55-152118. Further, in the AOD method, the ratio of dilution gas is increased in accordance with the decrease of a [C] concentration, or the vacuum refining is applied on the way of decarburization.
In one of the above methods and, for example, in the method using which the ratio of dilution gas is increased in accordance with the decrease of a [C] concentration, when the ratio of dilution gas is increased excessively in an attempt to prevent the oxidation of [Cr], this causes excessive consumption of an expensive dilution gas and therefore an increase in the refining cost. In contrast, when it is attempted to reduce the amount of dilution gas, the oxidation loss of [Cr] cannot be avoided sufficiently.
Further, when chromium-contained molten steel is subjected to decarburization-refining by the VOD method, the AOD method or the vacuum AOD method, it takes a long time for the measurement of a molten steel temperature and the sampling and chemical analysis of the molten steel. For that reason, in practice, the measurement of a molten steel temperature is performed intermittently or is not performed and the concentration of [C] in the molten steel is not analyzed continuously and, therefore, the refining operation conforming to a molten steel temperature and a [C] concentration may not be carried out sufficiently. As a consequence, in actual operation, PCO is lowered excessively, namely the degree of vacuum is increased excessively, or a dilution gas is injected abundantly, the oxidation of [Cr] and the erosion of refractories are not suppressed sufficiently, and thus the refining cost increases and the productivity lowers as a result of the decrease in an oxygen supply rate.
As a method for solving the problem of the molten steel temperature measurement among the aforementioned problems, a refining method has been described in Japanese Patent Publication No. H11-124618. Using this method, it is possible to measure the temperature of chromium-contained molten steel continuously, to control the ratio of oxygen gas to the injected gas and the addition amounts of alloys, coolants and auxiliary materials in accordance with the measured molten steel temperature, to reduce the oxidation loss of [Cr], and to mitigate the erosion of refractories of a refining furnace.
In this method, however, the transition of a decarburization oxygen efficiency (the rate of the blown oxygen gas consumed for decarburization, hereunder referred to as xe2x80x9cdecarburization oxygen efficiency xcex7xe2x80x9d) during blowing and a [C] concentration in molten steel are likely not obtained accurately, thus the control of refining in the operation under a reduced pressure is particularly insufficient, and, as a result, the oxidation loss of [Cr] is still large and the productivity is not improved sufficiently.
The present invention is provided to address the problems of the conventional technologies, such as insufficient suppression of the oxidation loss of [Cr] and the excessive erosion of refractories, in the decarburization-refining of chromium-contained molten steel under a normal or reduced pressure. One of the objects of the present invention is to reduce the oxidation loss of [Cr] and the erosion of refractories by determining the optimum refining conditions on the basis of a molten steel temperature, molten steel components and gas blowing conditions determined by actual measurement or assumption. Another object of the present invention is to reduce the oxidation loss of [Cr] and the erosion of refractories by using a means for measuring a molten steel temperature continuously, determining a decarburized amount continuously, estimating a [C] concentration in molten steel, and refining the molten steel in accordance with the molten steel temperature and the [C] concentration.
In a thermodynamic equilibrium, a Hilty""s equilibrium temperature, a Chipman""s equilibrium temperature and a Fuwa et al""s equilibrium temperature are known as a molten steel temperature T that balances with [C] and [Cr] concentrations (mass %) in molten steel and a CO partial pressure (PCO, atm.) in an atmosphere. Among those, a Hilty""s equilibrium temperature TH (K) is adopted in the present invention because it is widely used. A Hilty""s equilibrium temperature TH (K) is described by the following formula (5):
TH={xe2x88x9213,800/(xe2x88x928.76+log([C]PCO/[Cr]))}xe2x80x83xe2x80x83(5). 
It has been determined that, when refining was controlled so that the difference between a molten steel temperature T and a Hilty""s equilibrium temperature is approximately equal to a prescribed value or higher than the prescribed value in the refining of chromium-contained molten steel, specifically when refining was controlled so that PCO in an atmosphere and a molten steel temperature is adjusted on the basis of the transitions of [C] and [Cr] concentrations in the molten steel, the oxidation loss of [Cr] could be suppressed to the minimum and the erosion of refractories could be prevented without an excessive use of inert gas and an excessive increase of the molten steel temperature.
Further, in the control of refining based on a Hilty""s equilibrium temperature, the accuracy of the refining control improves as the accuracy of the estimation of [C] and [Cr] concentrations during the refining improves. A method according to the present invention is provided for estimating [C] and [Cr] concentrations with high accuracy.
According to one exemplary embodiment of the present invention, a method is provided for decarburization-refining chromium-contained molten steel under an atmospheric or a reduced pressure by blowing oxygen gas and inert gas into the molten steel a molten steel temperature is determined, in sequence, during the refining through actual measurement or computation from a molten steel temperature before the refining and refining conditions. [C] and [Cr] concentrations are determined during the refining through actual measurement or computation from molten steel components before the refining and refining conditions a CO partial pressure PCO in an atmosphere is determined during the refining from the total pressure P of the atmosphere. An oxygen gas supply rate and an inert gas supply rate is further determined. A Hilty""s equilibrium temperature TH is obtained from the [C] and [Cr] concentrations and PCO. The difference xcex94T between the molten steel temperature is determined during the refining. Further, the Hilty""s equilibrium temperature TH: and controlling the refining conditions are obtained so that the xcex94T is equal to a prescribed value or higher than the prescribed value.
The [C] and [Cr] concentrations can be determined during the refining through computation from molten steel components before the refining. In addition, a supplied oxygen amount can be determined as the total oxygen amount of oxygen gas and solid oxygen source, the transition of oxygen gas ratio to blown gas and the past refining data. The determinations of [C] and [Cr] concentrations during the refining through computation can be further performed with the analysis result of exhaust gas. Further, the [C] and [Cr] concentrations can be determined, in sequence, during the refining through the computation processes of: setting the [C] and [Cr] concentrations before the refining as the initial concentrations; and repeating the processes of determining a decarburization oxygen efficiency xcex7 as the function of the difference xcex94T. A decarburization rate and a [Cr] oxidation rate can be determined from the decarburization oxygen efficiency xcex7 and an oxygen gas supply rate, and the [C] and [Cr] concentrations may be revised.
The process of the decarburization-refining under a reduced pressure can be divided into three terms composed of a term from the commencement of decompression to the commencement of oxygen gas blow (a natural decarburization term), a term during which an oxygen gas ratio to blown gas is 20% or more after the natural decarburization term (an oxygen decarburization term), and a term during which an oxygen gas ratio to blown gas is less than 20% after the natural decarburization term (a diffusive decarburization term.
In the natural decarburization term, an [O] concentration is determined, in sequence, before the commencement of the decompression from a [C] concentration in the molten steel before the commencement of the decompression, a [C] activity (ac) in the molten steel and a molten steel temperature is determined before the commencement of the decompression; a decarburized amount is determined during the natural decarburization term as a function of the [O] concentration before the commencement of the decompression; and a [Cr] concentration is determined as a function of the decarburized amount during the natural decarburization term.
In the oxygen decarburization term, the [C] and [Cr] concentrations are determined, in sequence, during the refining through computation processes of: (i) setting the [C] and [Cr] concentrations at the time of the commencement of the oxygen decarburization term as the initial concentrations, and (ii) repeating the processes of determining a decarburization oxygen efficiency xcex7 as the function of said difference xcex94T, determining a decarburization rate and a [Cr] oxidation rate from the decarburization oxygen efficiency xcex7 and an oxygen gas supply rate, and revising said [C] and [Cr] concentrations.
In the diffusive decarburization term, the variation of the logarithmic value of the [C] concentration is determined from the commencement of the diffusive decarburization term as a function proportional to a time period t from the commencement of the diffusive decarburization term, and the [Cr] concentration is determined as a function of an oxygen supply rate and a decarburization rate.
According to another exemplary embodiment of the present invention, the following formula is used for determining a decarburization rate xcex94[C] and a [Cr] oxidation rate xcex94[Cr] from said decarburization oxygen efficiency xcex7 and an oxygen gas supply rate qT and revising [C] and [Cr] concentrations:
xcex94[C]=xcex7xc3x97qTxc3x97(1xe2x88x92R)xc3x9712/11.2/(10xc3x97Wm)xe2x80x83xe2x80x83(1), and 
xcex94[Cr]=(1xe2x88x92xcex7)xc3x97qTxc3x97(1xe2x88x92R)xc3x97104/33.6/(10xc3x97Wm)xe2x80x83xe2x80x83(2), 
where, xcex94[C] and xcex94[Cr] are the variations of [C] and [Cr] per unit time (mass %/min.), qT is an oxygen gas supply rate per unit time (Nm3/min.), R a secondary combustion rate (xe2x88x92), and Wm a molten steel amount (ton).
According to still another exemplary embodiment of the present invention, the CO partial pressure PCO in an atmosphere is determined during the refining from a total pressure P, an oxygen gas supply rate qT and an inert gas supply rate qd using the following formula (3):
PCO=Pxc3x972xc3x97qT/(2xc3x97qT+qd)xe2x80x83xe2x80x83(3). 
In a further exemplary embodiment, a value computed through the following formula is used as the Hilty""s equilibrium temperature TH:
TH={xe2x88x9213,800/(xe2x88x928.76+log([C]PCO/[Cr]))}xe2x80x83xe2x80x83(5), 
where, the units of the parameters are; K for TH, mass % for [C] and [Cr], and atm. for PCO.
In addition, refining conditions are determined so that xcex94T may be 0xc2x0 C. or more when a [C] concentration in the molten steel is 0.5 mass % or more, 30xc2x0 C. or more when the same is 0.2 mass % or more, and 50xc2x0 C. or more when the same is lower than 0.2 mass %. The xcex94T can be controlled by the control of a molten steel temperature when a [C] concentration in the molten steel is 0.5 mass % or more. The xcex94T can also be controlled by the control of the pressure in a refining vessel when a [C] concentration in the molten steel is in the range from 0.2 to 0.5 mass %. Further, the xcex94T may be controlled by the control of the pressure in a refining vessel and an oxygen gas ratio to blown gas when a [C] concentration in the molten steel is less than 0.2 mass %.
According to a further exemplary embodiment of the present invention, in the event of continuously measuring the temperature of said molten steel from the commencement of the decompression and continuously determining a decarburized amount (xcex94[%C]) and a [C] concentration ([%C]) by using the measured molten steel temperature, certain actions are taken. For example, in a term from the commencement of the decompression to the commencement of oxygen gas blow (a natural decarburization term), the following are determined in sequence: (i) an [O] concentration ([O]cal) before the commencement of the decompression from a [C] concentration ([%C]s) in the molten steel before the commencement of the decompression, a [C] activity (ac) in the molten steel and the measured molten steel temperature (T) before the commencement of the decompression, and (ii) a decarburized amount (xcex94[%C]) as a function of said [O]cal.
In addition, in a term during which an oxygen gas ratio to blown gas is 20% or more after the natural decarburization term (an oxygen decarburization term), the following are determined, in sequence: (i) a CO partial pressure PCO in an atmosphere at the time of the temperature measurement from the degree of vacuum (P) at the time of the temperature measurement, the total amount of oxygen gas (QT) blown during the temperature measurement span and the total amount of dilution gas (Qd) blown during the temperature measurement span; (ii) a Hilty""s equilibrium temperature from a computed [C] concentration ([%C]) in the molten steel, a computed [Cr] concentration ([%Cr]) in the molten steel and said PCO; (iii) the difference (xcex94T) between the measured molten steel temperature T and said determined Hilty""s equilibrium temperature; (iv) the ratio (xcex7) of oxygen gas consumed for decarburization to blown oxygen gas as a function of said xcex94T; the amount of oxygen gas (Q02) consumed for decarburization during the temperature measurement span from said xcex7 and QT and a secondary combustion ratio (R); and (v) a decarburized amount (xcex94[%C]) from said Q02 and a molten steel amount (Wm).
Further, in a term during which an oxygen gas ratio to blown gas is less than 20% after the natural decarburization term (a diffusive decarburization term), the variation of the logarithmic value (log[%C]) of the [C] concentration [%C] is determined from the commencement of the diffusive decarburization term as a function proportionate to a time period t from the commencement of the diffusive decarburization term.
According to still another exemplary embodiment of the present invention, in a term from the commencement of the decompression to the commencement of oxygen gas blow (a natural decarburization term), the following determinations are performed: (i) an [O] concentration ([O]cal) before the commencement of the decompression through the formulae {circle around (2)} and {circle around (3)} provided below; and a carburized amount (xcex94[%C]) through the following formula {circle around (1)} also provided below: (ii) in a term during which an oxygen gas ratio to blown gas is 20% or more after the natural decarburization term (an oxygen decarburization term), in sequence; a CO partial pressure PCO in an atmosphere at the time of the temperature measurement through the following formula {circle around (8)}; the difference (xcex94T) between the measured molten steel temperature T and said determined Hilty""s equilibrium temperature through the formula {circle around (7)} provided below; (iii) a ratio (xcex7) of oxygen gas consumed for decarburization to blown oxygen gas through the formula {circle around (6)} provided below; and (iv) a decarburized amount (xcex94[%C]) through the following formula {circle around (4)}: and 3) in a term during which an oxygen gas ratio to blown gas is less than 20% after the natural decarburization term (a diffusive decarburization term), determining the variation of the logarithmic value (log[%C]) of the [C] concentration [%C] through the formula {circle around (9)} provided below:
xcex94[%C]=axc3x97[O]cal+bxe2x80x83xe2x80x83{circle around (1)}, 
[O]cal=c+dxc3x97[O]exc3x9710fo/To+gxc3x97log[%C]s+hxe2x80x83xe2x80x83{circle around (2)}, 
[O]e=1/(acxc3x97fo)xc3x9710xe2x88x921,160/(To+273.15)xe2x88x922.003xe2x80x83xe2x80x83{circle around (3)}, 
xcex94[%C]=xcex7xc3x97QTxc3x97(1xe2x88x92R)xc3x9711.2/12/(10xc3x97Wm)xe2x80x83xe2x80x83{circle around (4)}, 
xcex7=Q02/((1xe2x88x92R)xc3x97QT)xe2x80x83xe2x80x83{circle around (5)}, 
xcex7=jxc3x97xcex94T+kxe2x80x83xe2x80x83{circle around (6)}, 
xcex94T=(T+273.15)xe2x88x92(xe2x88x9213,800/(xe2x88x928.76+log([%C]PCO/[%Cr])))xe2x80x83xe2x80x83{circle around (7)}, 
PCO=Pxc3x972xc3x97QT/(2xc3x97QT+Qd)xe2x80x83xe2x80x83{circle around (8)}, 
log[%C]xe2x88x92log[%C]O=mxc3x97txe2x80x83xe2x80x83{circle around (9)}, 
where,
T: measured molten steel temperature (xc2x0 C.),
To: molten steel temperature before the commencement of decompression (xc2x0 C.),
[%C]s: [C] concentration in molten steel before the commencement of decompression (mass %),
ac: activity of [C] in molten steel before the commencement of decompression,
fo: activity coefficient of [O] in molten steel before the commencement of decompression,
xcex7: ratio of oxygen gas consumed for decarburization to blown oxygen gas (xe2x88x92),
QT: total amount of oxygen gas blown during temperature measurement span (Nm3),
R: secondary combustion ratio (xe2x88x92),
Wm: molten steel amount (ton),
Q02: amount of oxygen gas consumed for decarburization during temperature measurement span (Nm3),
xcex94T: difference between actual temperature and Hilty""s equilibrium temperature (xc2x0 C.),
[%C]: computed [C] concentration in molten steel (mass %),
[%Cr]: computed [Cr] concentration in molten steel (mass %),
P: degree of vacuum at the time of temperature measurement (atm.),
Qd: total amount of dilution gas blown during temperature measurement span (Nm3),
[%C]0: [C] concentration in molten steel at the commencement of diffusive decarburization term (mass %),
t: time elapsed from the commencement of diffusive decarburization term (min.), and
a, b, c, d, f, g, h, j, k and m: constant values determined by a refining furnace and refining conditions.
In addition, PCO can be controlled so that xcex94T may be 50xc2x0 C. or more. PCO may also be controlled by the control of an oxygen gas ratio to blown gas when a [C] concentration in molten steel is 0.15 mass % or more. In addition, PCO can be controlled by the control of an oxygen gas ratio to blown gas and/or the control of an atmospheric pressure when a [C] concentration in molten steel is 0.15 mass % or less.
The entire disclosures of all publications referred to herein are incorporated herein by reference.